In an optical projection system, a beam of light illuminates a patterning element, and the pattern of light created by this element is cast onto a surface or volume in space. (The term “light” is used in the context of the present description and in the claims to refer to any sort of optical radiation, which may be in the visible, infrared and/or ultraviolet range.) The patterning element typically comprises a transparency through which the illuminating beam is transmitted, but in some cases it may comprise a reflective element. Projection systems are used in many applications, including three-dimensional (3D) mapping and imaging (also referred to as depth mapping) using structured or otherwise patterned light.
For example, U.S. Patent Application Publication 2010/0118123, whose disclosure is incorporated herein by reference, describes methods and systems for depth mapping using projected patterns. An illumination assembly includes a transparency containing a fixed pattern of spots. A light source transilluminates the transparency with optical radiation so as to project the pattern onto the object. An image capture assembly captures an image of the pattern that is projected onto the object. A processor processes the image captured by the image capture assembly so as to reconstruct a 3D map of the object.
In many illumination systems, it is desirable that the illuminating beam be as homogeneous as possible, with minimal variations in intensity over the field that is illuminated. Various means have been developed for beam homogenization. For example, U.S. Pat. No. 7,186,004 describes a homogenizing optical sheet, which accepts light transmitted at or within a specific entrance cone angle and then redirects and transmits the light within an exit cone that is substantially normal to the plane of the sheet. The intensity of the light within the exit cone is substantially uniform for any light source entering the sheet within the sheet's acceptance angle. The optical sheet is made of transparent material with microlens arrays formed on its opposite front and back surfaces. The thickness of the optical sheet is sufficient so that the microlenses on the opposite surfaces are separated a distance equal to the microlens focal length, with each microlens on the front and back surfaces having substantially similar size and shape, with centers transversely aligned.